Methods for magnetic imaging of geological structures

ABSTRACT

Methods for imaging geological structures include injecting magnetic materials into the geological structures, placing at least one magnetic probe in a proximity to the geological structures, generating a magnetic field in the geological structures and detecting a magnetic signal. The at least one magnetic probe may be on the surface of the geological structures or reside within the geological structures. The methods also include injecting magnetic materials into the geological structures, placing at least one magnetic detector in the geological structures and measuring a resonant frequency in the at least one magnetic detector. Methods for using magnetic materials in dipole-dipole, dipole-loop and loop-loop transmitter-receiver configurations for geological structure electromagnetic imaging techniques are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/350,914 filed Jan. 8, 2009, which claims priority to provisionalpatent applications 61/019,765 filed Jan. 8, 2008 and 61/054,362 filedMay 19, 2008, which are each incorporated by reference as if writtenherein in their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not applicable.

BACKGROUND

Most geological structures relevant to oil and gas production retainbetween 70% to 90% of their original hydrocarbon stores after primaryproduction driven by natural reservoir pressure release is complete.Hydraulic fracturing is often used to increase reservoir contact andincrease production rates. During the fracturing process, proppants aretypically added to a fracturing fluid pumped into the geologicalstructure in order to keep the fractures from closing in upon themselveswhen pressure is released. Another technique commonly used in secondaryproduction is displacement flooding, of which water-flooding is the mostcommon. In flooding techniques, a displacing fluid is introduced from aninjection well, and oil and/or gas are extracted from a nearbyproduction well. The displacing fluid frees oil or gas not releasedduring primary production and pushes the oil or gas toward theproduction well. Displacing fluids include, for example, air, carbondioxide, foams, surfactants, and water. Hydraulic fracturing is oftenapplied to injection and production wells in conjunction withdisplacement flooding operations.

In spite of the undisputed utility of hydraulic fracturing andwater-flooding in petroleum production processes, few methods exist formonitoring the extent and quality of the fracturing and floodingprocesses. Fractures can be monitored and approximately mappedthree-dimensionally during the fracturing process by a ‘micro-seismic’technique. The micro-seismic technique detects sonic signatures fromrocks cracking during the fracturing process. The setup of thistechnique is prohibitively expensive, and data that is generated tendsto be relatively inaccurate due to high background noise. Further, theprocess can only be performed during the fracturing process and cannotbe repeated thereafter. Water-flood operations can be monitored with lowresolution through four-dimensional seismic surveys. As the densitydifference between water and petroleum is small, the flood front is notabruptly distinguishable, and the imaging resolution tends to be on theorder of tens of meters. Unlike the micro-seismic technique formonitoring fracturing, flooding operations can be measured periodicallyto monitor flooding progression.

Neither of the above techniques have the capability to accuratelydetermine the size, structure and location of injected materials suchas, for example, injected proppants and water-flood. Improved knowledgeconcerning the location of injected proppants and water-flood infractures and natural geological pores would aid production engineers intailoring production conditions to meet local geological settings.Further, knowledge about the location of injected proppants andfractures would significantly improve safety in production processes byidentifying potentially catastrophic events before their occurrence. Forexample, vertical fractures can rupture the strata sealing geologicalstructures and potentially intersect fresh water aquifers. Detecting avertical fracture situation would allow production wells to be sealed,thereby preventing petroleum loss and aquifer damage.

In view of the foregoing, improved methods for imaging geologicalstructures are needed. Such methods would include the capability toobtain high-resolution images of fractures and injected materials, aswell as the ability for numerous measurement repetitions to be made.Utilizing such imaging methods solely or in combination with existinggeological assays, production engineers could take measures to extractresidual petroleum from a geological structure if it is determined thatun-extracted hydrocarbons remain after production stimulated byfracturing and flooding operations or a combination thereof is complete.

SUMMARY

In various embodiments, methods for assaying a geological structure aredisclosed. The methods include providing a dispersion of magneticmaterial in a fluid; injecting the dispersion of magnetic material intothe geological structure; placing at least one magnetic probe in aproximity to the geological structure; generating a magnetic field inthe geological structure with the at least one magnetic probe; anddetecting a magnetic signal.

In other various embodiments of methods for assaying a geologicalstructure, the methods include: a) providing a dispersion of magneticmaterial in a fluid; b) injecting the dispersion of magnetic materialinto the geological structure; c) placing at least one magnetic detectorinto the geological structure; and d) measuring a resonant frequency inthe at least one magnetic detector. The resonant frequency is at leastpartially determined by an amount of the magnetic material injected intogeological structure and a location of the magnetic material relative tothe at least one magnetic detector.

In other various embodiments, methods are disclosed for using magneticmaterials in electromagnetic imaging techniques utilizingtransmitter-receive antenna configurations such as dipole-dipole,dipole-loop and loop-loop configurations. An illustrative methodutilizing such transmitter-receiver antenna configurations includes, forexample, travel-time tomography.

The foregoing has outlined rather broadly various features of thepresent disclosure in order that the detailed description that followsmay be better understood. Additional features and advantages of thedisclosure will be described hereinafter, which form the subject of theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1 presents finite-element modeling of the radiofrequency amplituderesponse of a 1 Hz dipole placed over a brine-filled rock source (FIG.1A) and a brine-filled rock source loaded with 50μ_(o) of magneticmaterial (FIG. 1B);

FIG. 2 presents finite-element modeling of the extent of y-axismagnetization in the presence of a simulated 1 mG field generated by a 1Hz current loop, wherein the magnetic permeability is 1μ_(o) (FIG. 2A),5μ_(o) (FIG. 2B), 50μ_(o) (FIG. 2C) and 500μ_(o) (FIG. 2D);

FIG. 3 presents a 1:30 scale schematic model of the simulated magneticflux generated in a geological structure through a magnetic well-bore inthe absence of injected magnetic material;

FIG. 4 presents a 1:30 scale schematic model of the simulated magneticflux generated in a geological structure through a magnetic well-bore inthe presence of 50μ_(o) injected magnetic material; and

FIG. 5 presents finite-element modeling of simulated total magnetizationin a horizontal well-bore in the presence of 50μ_(o) injected magneticmaterial as determined by a resonant frequency magnetic detector withoffset (FIG. 5A) and non-offset (FIG. 5B) detector configurations.

DETAILED DESCRIPTION

In the following description, certain details are set forth such asspecific quantities, sizes, etc. so as to provide a thoroughunderstanding of the various embodiments disclosed herein. However, itwill be obvious to those skilled in the art that the present disclosuremay be practiced without such specific details. In many cases, detailsconcerning such considerations and the like have been omitted inasmuchas such details are not necessary to obtain a complete understanding ofthe present disclosure and are within the skills of persons of ordinaryskill in the relevant art.

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing particular embodimentsof the disclosure and are not intended to be limiting thereto. Drawingsare not necessarily to scale.

While most of the terms used herein will be recognizable to those ofordinary skill in the art, the following definitions are neverthelessput forth to aid in the understanding of the present disclosure. Itshould be understood, however, that when not explicitly defined, termsshould be interpreted as adopting a meaning presently accepted by thoseof skill in the art.

“COMSOL®,” refers to a finite-element modeling (FEM) software packageavailable for various physics and engineering applications(http://www.comsol.com). COMSOL® modeling presented herein includesstatic and time-varying three-dimensional electromagnetic modeling.

“Ferrite” as defined herein, refers to a ferromagnetic compound formedfrom iron (III) oxide and another oxide. Illustrative ferrites includematerials with a general formula AM₂O₄, wherein A and M are metal atomsand at least one of A and M is Fe.

“Ferrofluid,” as defined herein, refers to a liquid that becomespolarized in the presence of a magnetic field. A ferrofluid typicallyincludes a paramagnetic, superparamagnetic, ferromagnetic orferrimagnetic material disposed as a colloidal suspension in a carrierfluid such as, for example, an organic solvent or water. The magneticmaterial disposed in the carrier fluid can be a magnetic nanoparticle.

“Hematite,” as defined herein, refers to a common mineral form of iron(III) oxide.

“Magnetite,” as defined herein, refers to a ferrimagnetic mineral havinga chemical formula Fe₃O₄.

“RLC circuit,” as defined herein, refers to an electrical circuitincluding a resistor (R), an inductor (L), and a capacitor (C),connected in series or in parallel.

Most economically interesting geological structures such as, forexample, petroleum reservoirs, have low magnetic permeabilities,essentially equal to that of vacuum, μ_(o). In various embodiments, thepresent disclosure describes injecting magnetic materials intogeological structures and then detecting the magnetic materials withinthe geological structures. Detecting the magnetic materials provides ameans for imaging the locations of fractures and injected materialswithin the geological structures. Since the magnetic permeabilities ofgeological structures are typically very low, any injected magneticmaterials will substantially modify detected magnetic flux responsecompared to that typically seen for native rocks, natural gas, oil,water, and brine of most geological structures. Such magnetic imagingtechniques are advantageous over methods currently in use for monitoringpetroleum production by allowing high-resolution and repeatable imagingduring production processes.

In various embodiments, methods for assaying a geological structure aredisclosed. The methods include providing a dispersion of magneticmaterial in a fluid; injecting the dispersion of magnetic material intothe geological structure; placing at least one magnetic probe in aproximity to the geological structure; generating a magnetic field inthe geological structure with the at least one magnetic probe; anddetecting a magnetic signal. In some embodiments, the geologicalstructure is penetrated by at least one vertical well. In someembodiments, the geological structure is penetrated by at least onehorizontal well. One skilled in the art will recognize that the termsvertical well and horizontal well should not be considered limiting, andvarious well-bore angles between these two extremes are common in theart and may be utilized within the spirit and scope of this disclosure.In various embodiments, the geological structure includes a deposit suchas, for example, oil, gas, or combinations thereof.

Geological structures have been characterized over geologically-relevantdimensions using electromagnetic methods, but these methods have notheretofore utilized injected magnetic materials. Electromagnetic methodsfor characterizing geological structures have typically relied upon thelow conductivity and permittivity of petroleum compared to brine, whichis usually found concurrently with petroleum in geological structures.An illustrative electromagnetic method for characterizing geologicalstructures is controlled-source electromagnetic (CSEM) surveying. Inthis method, variations in geological structure conductivity aredetected via the electrical component of an applied electromagneticfield. Spatial variation in conductivity results in changes in receivedsignal amplitude, thus indicating a possible petroleum-containinggeological structure. CSEM surveying has typically been used for mappingnon-conductive deposits in deep marine environments using electricdipole transmitter and receiver antennas. The thick layer of highlyconductive seawater screens the electric dipole receiver antennas fromair-path interference.

The effects of injected magnetic material may be simulated usingfinite-element modeling, such as that performed using COMSOL® software.Illustrative finite-element modeling depicting the influence of injectedmagnetic materials on radiofrequency amplitude response in the presenceof a 1 Hz dipole in proximity to a simulated geological structure areshown in FIGS. 1A and 1B. FIG. 1A presents finite-element modeling ofradiofrequency amplitude response of a 1 Hz dipole over a brine-filledrock source. FIG. 1B presents finite-element modeling of radiofrequencyamplitude response of a 1 Hz dipole over the same brine-filled rocksource loaded with 50μ_(o) of magnetic material. In the illustrativemodels presented in FIGS. 1A and 1B, the target measurement zone islocated on-shore with dimensions of 4 kilometers long by 200 meters wideand positioned 1000 meters below a rock/air interface within a spherehaving a radius of 5 kilometers. The target measurement zone is modeledas rock filled with brine having a conductivity σ=1.5 S (FIG. 1A) andbrine loaded with magnetic material at μ=50μ_(o) (FIG. 1B). As isevident from comparing the magnetic flux lines in FIGS. 1A and 1B,magnetic material in the target measurement zone significantly spreadsradiofrequency amplitude response at the air/rock interface.

The amount of injected magnetic material significantly influences thespread of magnetic flux lines as illustrated by the models shown in FIG.2. FIGS. 2A, 2B, 2C and 2D illustrate finite-element modeling depictingthe extent of y-axis magnetization in the presence of a simulated 1 mGfield strength generated by a 1 Hz current loop. The plane of thecurrent loop is oriented vertically. The current loop is 2 meters indiameter and embedded in a target plane of 30% porosity rock filled withbrine. The target plane is 1000 meters below the surface of a geologicalformation, above which is air. The target plane magnetic permeability isvaried through 1, 5, 50 and 500μ_(o) to illustrate the change inobserved y-axis magnetization. As is evident from FIGS. 2A, 2B, 2C and2D, the extent of y-axis magnetization is highly influenced by theamount of magnetic material present. Similar changes in magnetizationcan be visualized along other axes.

A number of different magnetic materials may be used in the methodsdescribed herein. Magnetic materials of the present disclosure typicallyare characterized by high magnetic permeabilities at low appliedmagnetic fields. Low applied magnetic fields typically include, forexample, magnetic field strengths less than about 0.1 Tesla. One skilledin the art will recognize, however, that higher magnetic fields may beapplied in the methods described herein. Magnetic materials include, forexample, paramagnetic, superparamagnetic, ferromagnetic andferrimagnetic materials. In various embodiments, the magnetic materialsare dispersed in a fluid such as, for example, water, brine, drillingmud, fracturing fluid and combinations thereof. In some embodiments, thefluid includes a proppant such as, for example, sand. Injection ofmagnetic materials can be conducted during fracturing or floodingoperations. Magnetic materials can be added to injected proppants andused during fracturing to monitor the extent of the fracturing process.Likewise, magnetic materials can also be used during flooding operationsto monitor flood front progression through the geological structure.

In various embodiments of the methods, the dispersion of magneticmaterial comprises a ferrofluid. The ferrofluid may include a dispersionof magnetic nanoparticles, which forms the ferrofluid. Ferrofluids maybe injected directly into the geological structures or diluted inanother fluid for injection in the geological structures. In variousembodiments of the methods, the magnetic material comprises magneticnanoparticles. In various embodiments of the methods, the magneticmaterial includes, for example, iron, cobalt, iron (III) oxide,magnetite, hematite, ferrites, and combinations thereof. As definedhereinabove, an illustrative ferrite has a general chemical formulaAM₂O₄, where A and M are metal atoms and at least one of A and M is Fe.In various embodiments, the ferrites are doped with at least one elementthat is not A or M. Ferrofluids generally include magnetic metal ormetal oxide particles such as, for example, iron, cobalt, iron (III)oxide, and magnetite. Several ferrofluids are commercially available orare easily synthesized. Most commercially-available ferrofluids arebased on magnetite, which provides a low-field permeability of about100μ_(o). Higher permeabilities are advantageous for increased detectionsensitivities in the embodiments described herein. An illustrativehigh-permeability ferrofluid is formed from a manganese- and zinc-dopedferrite, which provides a low-field permeability of about 25,000μ_(o).Doping a barrel of brine to about 50μ_(o) would require about 160 gramsof this doped ferrite. Based on current prices of iron, manganese andzinc, brine doping could be accomplished for at most a few dollars perbarrel, making the methods described herein economically viable forgeological structure assays. Iron nanoparticle suspensions and simpleslurries of iron powders having grain sizes similar to those of sand arealso commercially available and are suitable for use in the methodsdescribed herein.

In various embodiments, the magnetic materials are modified prior totheir injection into the geological structures. Modifications are used,for example, to prevent particle aggregation in the injection fluid, toreduce adhesion to the geological structures, and to maximize transportthrough the geological structures. In various embodiments, the magneticmaterials are covered with a coating such as, for example, surfactants,polymers and combinations thereof. Surfactants are selected fromneutral, anionic, or cationic surfactants.

The sizes of the injected magnetic materials are chosen to be mostcompatible with the selected magnetic imaging application. Typicalproppants used in hydraulic fracturing operations nominally resemblesand grains having diameters between about 300μm to about 1 mm.Hydraulic fractures, in comparison, can be about one centimeter wide orgreater. Naturally-occurring pores in geological structures encompass awide range of dimensions depending on local rock types and degrees ofcementation. Pores in typical sandstones are in the range of about 10μmto about 50μm. Carbonates typically have a wide pore size distributionranging between about 100 nm and about 10 mm. The bulk of the porevolume in common petroleum-producing rocks includes pores typicallygreater than about 100 nm in diameter. Therefore, magnetic materialsused in the methods described herein may be varied through a wide rangeof sizes to be compatible with natural pore sizes and fractures. Invarious embodiments of the methods disclosed herein, the magneticmaterials have diameters between about 10 nm and about 1μm. In variousembodiments of the methods disclosed herein, the magnetic materials havediameters between about 10 nm and about 100 nm. In various embodimentsof the methods disclosed herein, the magnetic materials have diametersbetween about 10 nm and about 50 nm. As will be evident to one skilledin the art, particle size of the magnetic material is chosen whilebearing factors other than geological structure pore size and fracturesize in mind. For example, magnetic material particle size can influencethe particle's observed magnetic properties, hydrodynamic radius,aggregation tendency, and extractability.

Magnetic fields may be generated within geological structures throughvarious means by using a magnetic probe. For example, the magneticfields can be supplied by permanent magnets, electromagnets, solenoids,antennas and combinations thereof. The magnetic probe produces amagnetic field that may be a DC field, an AC field, a pulsed field, or afield that varies in both time and amplitude. The magnetic probe fieldmay be modulated in a manner to enable frequency-domain, time-domain orphase-shift detection methods to maximize signal-to-noise ratio, and tomaximize rejection of natural background noise and 1/f noise.

Magnetic fields are projected in the geological structures in a numberof ways by using a magnetic probe. An illustrative means for generatinga magnetic field in the geological structures involves well-borespenetrating the geological structures. In various embodiments, thegeological structures are penetrated by at least one well comprising aferromagnetic material, and the ferromagnetic material is used tochannel a magnetic field into the geological structures. Steels commonlyused in drill stems and well-bore casings are typically stronglyferromagnetic with a low-field permeability up to about 5,000μ_(o).Connecting a magnetic probe magnetization source such as, for example, apermanent magnet or solenoid at the surface end of such well-borecasings allows a magnetic field component, B, to be transmitted alongthe well-bore casing into the geological structures. The well-borecasing therefore provides a magnetic flux distal to the magnetizationsource. When utilized in this manner, the well-bore casings functionanalogously to an antenna for transmitting a magnetic probe signal intothe geological structures.

FIG. 3 presents a 1:30 scale schematic model of the simulated magneticflux generated in a pristine geological structure 300 (no native orinjected magnetic material) using a magnetic well-bore 302. Thelogarithm of the magnetic field intensity is indicated by color contourin the figure. A magnetic probe magnetization source (not shown) isapplied to surface end 301 of well-bore 302. The model includes emptyinjection zone 303 to be used for introducing magnetic material. Forthis illustrative model, well-bore 302 was chosen to be 100 meters indepth, and injection zone 303 was chosen to be 5 meters in thickness.Magnetic flux lines 304 are measured using movable detector 305, whichis transported along surface 306 of pristine geological structure 300.Magnetic flux lines 304 are illustrative of those obtained in theabsence of injected magnetic material.

FIG. 4 presents the same 1:30 scale schematic model of the simulatedmagnetic flux generated in infiltrated geological structure 400 afterinjecting sufficient magnetic material into flooded injection zone 401to produce a permeability of about 50μ_(o). As in FIG. 3, magnetic fluxlines 404 are measured using movable detector 405, which is transportedalong surface 406 of infiltrated geological structure 400. Comparison ofthe magnetic flux lines 404 in FIG. 4 to the magnetic flux lines 304 inFIG. 3 demonstrates that substantial changes in simulated magnetic fluxcan be realized by injecting magnetic materials into the geologicalstructures. The models presented in FIGS. 3 and 4 are equally valid foruse in operations involving magnetic material-doped water-flood ormagnetic material-loaded proppant.

The methods illustrated by the models presented in FIGS. 3 and 4 areadvantageous in being applicable to both fracturing and water-floodstages of the petroleum production process. Further, the methods arereadily repeatable to monitor production in near real-time, factoringinto account integration time length for data acquisition andprocessing. In various embodiments, the detected magnetic signal iscorrelated with an internal structure of the geological structure. Forexample, changes in the magnetic flux lines are indicative of internalstructure alterations of the geological structure. Monitoring of changesto the internal structures of the geological structure allows productionmonitoring. In various embodiments, the methods include detecting amagnetic signal in the geological structure before injecting thedispersion of magnetic material. Measuring a magnetic signal in thegeological structure before production begins provides a baseline forevaluating internal structure changes resulting from fracturing orwater-flood operations. For example, comparison via subtraction can beutilized to analyze the geological structure before and after productionbegins. In typical practice of the methods described herein, productionengineers will compare field magnetization data with mathematical modelssuch as, for example, those presented hereinabove and related inversiontechniques to infer the size, shape, and extent of magnetic materialincursion in the geological structure.

A magnetic signal may be induced at the surface or below the surface ofthe geological structure. In various embodiments, the proximity of theat least one magnetic probe is above the geological structure. Forexample, as discussed hereinabove, a magnetic field can be projectedinto the geological structure through a magnetic well-bore. In othervarious embodiments, the proximity of the at least one magnetic probe iswithin the geological structure. For example, a magnetic field can begenerated within the geological structure with a solenoid located withinthe geological structure. Magnetic fields generated within thegeological structure are particularly useful for practicing the methodsdescribed herein when there is no magnetic well-bore penetrating thegeological structure.

Detection of magnetic flux lines may be conducted at one or moredetection points away from the magnetic probe providing the appliedmagnetic field. Detection may occur on the surface of the geologicalstructure or within the geological structure. Detection may beaccomplished with a single detector or an array of detectors. Detectorsmay be stationary or movable to record magnetic flux data at more thanone point. In various embodiments, the detecting step is conducted withat least one detector that is movable. In various embodiments, thedetecting step includes detecting a magnetic signal, moving the at leastone detector, and repeating the detecting step to collect magnetic fluxdata at more than one point. Detector arrays are used to record magneticsignals at a number of points simultaneously. A single detector may be,for example, a SQUID detector or a conventional solenoid, each of whichmay be fixed or movable over the surface of the geological structure. Invarious embodiments, the detecting step is conducted with at least oneSQUID detector. SQUID detectors are advantageous for maximizingsensitivity in the methods described herein. In other variousembodiments, the detecting step is conducted with at least one solenoid.For detector arrays, low cost conventional solenoids or other knownmagnetic sensors are more advantageous. In still other variousembodiments, the detecting step includes measuring a resonant frequencyin the at least one magnetic probe. Measurement of a resonant frequencyin the at least one magnetic probe provides a particularly sensitivemeans of magnetic permeability detection and is considered in moredetail hereinbelow.

Not all drilling applications involve a vertical well-bore as depictedin FIGS. 3 and 4. For example, horizontal drilling techniques provide‘lateral’ well-bores. Such lateral well-bores are typically used tomaximize contact with a geological structure. Lateral well-bores may beoptionally fractured prior to or during production or used inconjunction with water-flood.

The orientation of lateral well-bores may not allow sufficientchanneling of an external magnetic field into the geological structure,even when the lateral well-bore includes a ferromagnetic well casing. Insuch instances and others, at least one magnetic detector may be placedinto the well-bore to measure the magnetic flux lines. At least onemagnetic detector may also be placed in a vertical well-bore when anexternal magnetic field is not sufficiently channeled into thegeological structure from above using a magnetic probe. In other variousembodiments of methods for assaying a geological structure, the methodsinclude: a) providing a dispersion of magnetic material in a fluid; b)injecting the dispersion of magnetic material into the geologicalstructure; c) placing at least one magnetic detector into the geologicalstructure; and d) measuring a resonant frequency in the at least onemagnetic detector. The resonant frequency is at least partiallydetermined by an amount of the magnetic material injected intogeological structure and a location of the magnetic material relative tothe at least one magnetic detector. In various embodiments, the at leastone magnetic detector is connected to an RLC circuit. In variousembodiments, the methods include measuring a resonant frequency, movingthe at least one magnetic detector, and repeating the measuring step. Aresonant frequency magnetic detector may include, for example, asolenoid or a directional-loop antenna placed within a well-bore. Whenconnected to an active RLC circuit, a resonant frequency of a solenoidis determined by the capacitance and inductance of the C and L circuits,respectively. Positioning of the solenoid coil within the geologicalstructure provides an inductance that is at least partially determinedby the amount of magnetic material that is injected into the geologicalstructure and the location of the magnetic material relative to thesolenoid coil.

FIG. 5 presents finite-element modeling of the simulated totalmagnetization in a lateral well-bore as determined by a resonantfrequency magnetic detector with an offset (FIG. 5A) and non-offset(FIG. 5B) detector configuration relative to a high-flow rate path ornatural fracture channel (herein referred to as a ‘runner’) away fromthe main well-bore. Effects of runners in petroleum production include,for example, early water breakthrough, which impedes efficient petroleumproduction. Therefore, timely detection of runners is clearly desirable.In the finite-element models presented in FIGS. 5A and 5B, thegeological structure is injected with 50μ_(o) of magnetic material, andthe magnetization is determined at a detector offset of 25 meters fromthe runner in FIG. 5A and zero meters from the runner in FIG. 5B. Themagnetic flux line changes produced with centered and offset detectorconfigurations are illustrative of the changes observable in physicalgeological structures penetrated with magnetic materials duringfracturing or water-flood operations. Filling of the runner withmagnetic material changes the total magnetization, thereby changing L byabout one part in 10⁴, as the magnetic probe position is moved away fromthe runner. Such frequency changes are typically measurable to withinabout one part in 10⁹ or greater using electronic detectors such as, forexample, frequency counters. Thus, in geological structures injectedwith magnetic materials, the location of a flood front, includingprogression into runners and filled fractures, may be detected using amoveable resonant frequency magnetic detector. In dual well-boresystems, the movable resonant frequency magnetic detector may be placedin the injection well-bore, production well-bore, or both well-bores.Likewise, the movable resonant frequency magnetic detector may belocated on the surface of the geological structure.

Dipole-dipole, dipole-loop and loop-loop configurations for cross-welland borehole-to-surface electromagnetic imaging techniques have beenunder development for some time for detecting and imaging conductivesubsurface features. Generally, frequency (phase) domain detection andinversion have been employed in such systems, despite their highcomputational intensity. Forward-propagation and inversion algorithmsfor cross-well electromagnetic propagation in diffusive media have alsobeen under development. A relatively new time-domain approach calledtravel-time tomography potentially provides simpler electronics andreduced computational burden relative to other techniques in this field.Magnetic imaging techniques have not yet been applied in these moreadvanced geological imaging surveys. In various embodiments, methods aredisclosed for using magnetic materials in electromagnetic imagingtechniques utilizing transmitter-receiver antenna configurations suchas, for example, dipole-dipole, dipole-loop, and loop-loopconfigurations. More sensitive detectors including, for example,flux-gates and SQUID detectors may also be coupled to theelectromagnetic imaging techniques.

The methods disclosed herein for using magnetic materials indipole-dipole, dipole-loop and loop-loop configurations includedevelopment of approaches for inverting time and amplitude signals inthe presence of magnetic materials and an estimation of computationalintensity needed for each. An additional frequency domain detectiontechnique relevant to these approaches is referred to as higher-orderspectral analysis. The higher-order spectral analysis techniques includethe use of the coherence of multiple frequency components to detectweaker signals in the presence of Gaussian and non-Gaussian noise. Amulti-frequency coherent source or a scattering mechanism that is eithernonlinear or parametric-linear is used in application of the techniques.Application of magnetic materials in these techniques is advantageous inproviding signal detection (with sufficient averaging) in the presencehigher noise levels than is possible with more traditional frequencydomain detection techniques. Further, application of magnetic materialsin the techniques allows sensor arrays to be used to determine time ofarrival for inversion processing at lower signal to noise ratios.

A consideration concerning the use of magnetic materials in imaginggeological structures is a modulation of electromagnetic signaltransport. In a magnetically-loaded fluid, electromagnetic signal speeddecreases in accordance with the formula (I), where c is the speed

v=c/(∈μ)^(1/2)  (1)

of light, ∈ is the relative dielectric constant, and μ is the relativemagnetic permeability. Velocity changes in electromagnetic signals havebeen used extensively in the art of well logging, where the dielectricconstant difference between petroleum and water shifts observed signalvelocity from 5 ns/m to 29 ns/m. Electromagnetic signal velocity in anaqueous ferrofluid at 50μ₀ is calculated to be about 200 ns/m.Therefore, a significant electromagnetic signal velocity shift ispossible for magnetically-loaded water-floods when applied totravel-time tomography imaging. Higher-order harmonics generated uponsaturation of the magnetic materials would facilitate frequency domaindiscrimination such as, for example, through higher-order spectralanalysis.

Any of the methods described hereinabove are potentially applicable fortracking pollutants within a geological structure. For example, leakagefrom a chemical storage facility could potentially be monitored byadding a magnetic material at the chemical storage facility source andthen analyzing for the presence of magnetic material in a nearbygeological structure. An abrupt or gradual change in magnetic signalwould indicate a leaking condition Likewise, the methods couldpotentially be used to monitor pollutant migration through a geologicalstructure such as, for example, from agricultural runoff. Similarly, themethods described herein could potentially be used to monitor thetransport and chemical conversion of zero-valent iron particles that areused in ground water contamination remediation.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this disclosure, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications to adapt the disclosure to various usages andconditions. The embodiments described hereinabove are meant to beillustrative only and should not be taken as limiting of the scope ofthe disclosure, which is defined in the following claims.

1. A method for imaging a geological structure, the method comprising:providing a dispersion of magnetic material in a fluid, wherein themagnetic material comprises magnetic nanoparticles; injecting a fluidinto the geological structure; placing at least one magnetic probe in aproximity to the geological structure; generating a magnetic field withthe at least one magnetic probe; and detecting a post-injection magneticsignal.
 2. The method of claim 1, wherein the magnetic probe provides anantenna utilizing a dipole-dipole, dipole-loop, or loop-loopconfiguration.
 3. The method of claim 1, wherein the magnetic probe isan electromagnetic probe that generates an electromagnetic field.
 4. Themethod of claim 3, further comprising detecting an electromagneticsignal, wherein the electromagnetic signal detected is utilized to imagethe geological structure.
 5. The method of claim 3, further comprisingdetecting an electromagnetic signal velocity.
 6. The method of claim 1,wherein the post-injection magnetic signal detected is utilized to imagefractures in the geological structure.
 7. The method of claim 1, furthercomprising detecting a prior magnetic signal, wherein the prior magneticsignal is detected before injecting the dispersion of magnetic materialinto the geological structure.
 8. The method of claim 7, wherein theprior magnetic signal is compared to the post-injection magnetic signalto analyze the geological structure.
 9. The method of claim 1, whereinthe magnetic field is a DC field, an AC field, a pulsed field, or afield that varies in both time and amplitude.
 10. The method of claim 1,wherein the magnetic field generated by the magnetic probe is modulatedto enable frequency-domain, time-domain, or phase-shift detection. 11.The method of claim 1, wherein the detecting step comprises measuring aresonant frequency.
 12. The method of claim 1, wherein the dispersion ofmagnetic material in the fluid provides a permeability of 50μ_(o) orgreater.
 13. The method of claim 1, wherein the dispersion of magneticmaterial comprises a ferrofluid.
 14. The method of claim 1, wherein themagnetic material is selected from the group consisting of iron, cobalt,iron (III) oxide, magnetite, hematite, ferrites, and combinationsthereof.
 15. The method of claim 14, wherein the ferrites comprise amaterial having a formula AM₂O₄; wherein A and M are metal atoms; andwherein at least one of A and M are Fe.
 16. The method of claim 15,wherein the ferrites are doped with at least one element that is not Aor M.
 17. The method of claim 1, wherein the magnetic material has adiameter of between 10 nm to 1μm.
 18. The method of claim 1, wherein themagnetic material is covered with a coating selected from the groupconsisting of surfactants, polymers, or combinations thereof.
 19. Themethod of claim 1, wherein the fluid is selected from the groupconsisting of water, brine, drilling mud, fracturing fluid, andcombinations thereof.
 20. The method of claim 1, wherein the detectingstep is conducted with at least one SQUID detector.